Ultrasonic techniques are commonly used in medical imaging systems to study the anatomy and function of organs and other tissue structures within the body. Such systems typically energize a transducer to transmit short pulses of ultrasound into the body. The backscattered ultrasonic energy reflected by acoustic interfaces within the body is converted by the transducer into an electrical signal. The amplitude of the signal at various points in time is detected and this information is utilized to construct a moving image representing a tomographic slice through parts of the body. Such systems are also capable of obtaining information about the direction and velocity of blood flow within the body utilizing Doppler techniques. Referring to FIG. 1, a curve 8 represents the frequency spectrum transmitted by the transducer. The Doppler effect resulting from ultrasound striking moving red blood cells manifests itself as a shifting of the entire frequency spectrum upwardly or downwardly without a significant change in overall spectrum shape, as illustrated by the curves 9a and 9b of FIG. 1. This shift is detected and, in Doppler color flow mapping systems, the imaging system causes different colors and intensities to be superimposed on the moving image based upon the detected shift so that an indication of direction and velocity of blood flow is obtained. A different type of system known as a spectral Doppler display system creates a graphical display of blood flow velocity and direction plotted against time.
More recently, ultrasound contrast agents have been developed to allow the study of perfusion or distribution of blood supply within body tissues. Such contrast agents are commonly made of small microbubbles or gas filled spheres. Such contrast agents are strong scatters of ultrasound. Hence, if they are injected or delivered into the blood supply of an organ or other tissue, their passage therethrough can be detected by examining the increase in backscattered ultrasonic intensity using standard ultrasound imaging equipment like that described above. Semi-quantitative assessment of degrees of perfusion may be obtained using additional equipment that can analyze the magnitude of increase in backscattered intensity. Also, various temporal parameters can be measured which relate to the number of blood cells and contrast microbubbles flowing through specific areas of tissues. Such systems have the disadvantage, however, in that a large number of contrast microbubbles must be delivered into the tissue to provide a sufficient change in backscattered intensity that can be reliably detected.
Recent improvements in the production of contrast agents has led to the development of microbubbles of an acceptably consistent size on the order of the size of red blood cells or smaller. Such microbubbles can travel through the lungs into the arteries following an intravenous injection and hence these contrast agents can reach organ tissues without the need to perform arterial catheterization. While the use of such contrast agents involves less risk and is less expensive and more convenient to use than agents that must be delivered via catheterization into an artery, it appears that the number of contrast microbubbles reaching organ tissues following intravenous injection is insufficient to permit reliable evaluation of tissues using changes in backscattered intensity. In addition, variable attenuation of the ultrasound signals by body tissues and the contrast agent in the space between the transducer and the tissues to be studied limits the use of methods reliant upon changes in backscatter intensity for evaluation of relative perfusion.